THE EYE 



1051 



associated with the formation of the face and nose (see Morphogenesis). The cells of the sur- 

 face epithelium on the olfactory pits in part form olfactory cells, and send processes inward which 

 pass to the olfactory lobe of the brain, and form the olfactory nerve. 



The organ of jacobson is a small rudimentary structure in man. It is represented by a 

 minute canal, 2 to 9 mm. Ions, placed on each side in the lower portion of the nasal septum, 

 opening on the surface slightly above the orifice of the naso-palatine canal. Below it lies a 

 small piece of cartilage, lying below the cartilage of the septum, and known as Jacobson's carti- 

 lage. The canal is lined by epithelium, but contains no olfactory cells. It is developed from 

 a small portion of the olfactory plate which becomes separated from the area which gives rise 

 to epithelium of the olfactory region. 



II. ORGAN OF TASTE 



The taste organs [organon gustus] consist of minute epithelial structures, the 

 taste buds [calyculi gustatorii], situated mainly in the epithelial covering of the 

 tongue and also in the epiglottis. 



In the tongue, the ta.ste buds are found mainly on the walls of the vallate 

 papillae (see p. 1106), but they are found to a slight extent scattered over the 

 whole area of distribution of the glosso-pharyngeal nerve, on the surface of the 

 foliate and fungiform papillae, and on the plicae fimbriatae on the lower surface of the 

 tongue. 



Figs. 794 and 795. — Diagrams Illustrating the Structure of the Taste Buds 

 AND the Olfactory Mucosa. 



Supporting Gustatory 

 cells cells 



In the foetus, the distribution is even wider, and they have been described as occurring on 

 the soft palate, palatine arches, uvula, and in the mucous membrane covering the medial surfaces 

 of the arytenoid cartilages. It is possible that such structures, though found in these regions 

 in the foetus, usually disappear in the adult. 



Each taste bud is a hollow conical or oval structure, measuring .07-.08 mm. 

 in length. At one end it opens by a small channel, termed the pore canal, which 

 passes to the surface between adjacent epithelial cells. The surface opening is 

 termed the outer pore and the opening at the taste bud the inner taste pore. 



The taste bud consists of epithelial supporting, of gustatory and of basal cells, arranged as 

 seen in figure 794. The gustatory cells are long slender fugiform cells. The free end of 

 each passes to the inner taste pore, and terminates in stiff hair-like processes, which project 

 toward the pore canal. The deep end of each is connected with a basal cell. Terminal branches 

 of the glosso-pharyngeal nerve ramify around the gustatory cells, and convey to the brain the 

 impulses generated by contact of the ends of these cells with sapid particles. The epithelial 

 supporting cells line the taste buds, and also project into the interior between the olfactory cells. 



Development. — The taste buds appear comparatively late in embryonic life — about the 

 third month. They arise mainh^ from the entodermal portion of the tongue, by differentiation 

 of the deeper cells of the epithelial covering over localised areas. Around these cells terminations 

 of the glosso-pharyngeal nerve are found. These cells assume the characteristic shape and 

 arrangement of the adult to form a taste bud. At first the opening of the bud lies upon the 

 surface, but as the surrounding epithelial cells increase in size and thickness, the pore-canal is 

 formed as a space between adjacent epithelial cells on the summit of the bud. 



III. THE EYE 



The sensory portion of the eye is the retina, a cup-shaped membrane, which 

 lines the posterior half of the eyeball. It is formed of layers of nerve cells, from 



